The present invention relates to a method for detecting multipath signals, in particular for operating a RAKE receiver and further relates to an associated RAKE receiver.
Signals generally propagate over two or more paths in one mobile radio channel (multipath propagation). Each individual received signal has a different propagation time, amplitude and Doppler shift, corresponding to its transmission path. The various received signals are superimposed at the antenna of a receiver either constructively or destructively, thus making it harder to detect the symbols transmitted via the signals.
In particular, the multipath propagation of a radio signal such as this results in channel impulse responses over an extended time, which lead to a delay spread and thus to a shifted time reference between the individual symbols in the transmission signal. In mobile radio systems which operate using the Wideband Code Division Multiplex (W-CDMA) method, this then requires that special receivers (RAKE receivers) be used for processing the multipath signals.
A RAKE receiver such as this allows a considerable gain as a result of the use of multipath signals which arrive with different propagation time delays at a receiving antenna of a mobile radio. This is done by the signal coming from the antenna being processed in the receiver in two or more paths, the so-called “fingers” of the RAKE receiver. Each of these fingers is set, in each case, with an optimized phase angle of the pseudo-noise sequences which have been used for the coding of a signal to be transmitted via the mobile radio channel, to one respective component of the received multipath signals.
In an IS-95 mobile radio system, a RAKE receiver in a mobile station has at least three such fingers, and a base station has at least four such fingers. Furthermore, both the mobile station and the base station each have at least one “search finger” which continuously looks for stronger multipath signals. As soon as the search finger finds a stronger multipath signal, the finger on the previously weakest multipath signal is optimally set to the new, stronger multipath signal. Typically, this makes it possible to demodulate up to three of the strongest multipath signals in the mobile station and up to four of the strongest multipath signals in the base station, with a time delay of at least 0.8 to 1 μs, and to combine them via maximum ratio combining.
In this case, it is particularly important to detect the time reference between the individual multipath signals exactly; that is, to define the optimum “finger position”. A specially tuned filter, a so-called matched filter, thus uses a sequence or a pilot sequence which is known in the receiver to produce the overall impulse response or channel impulse response of a received signal, on the basis of which the position of the individual fingers can be determined. However, the overall impulse response does not always allow detection of all the relevant finger positions. Particularly when adjacent fingers differ, for example, only by a very short delay time of less than the chip duration, reliable detection of the position and of the number of the fingers is difficult.
Known methods for detection of the finger positions are, however, highly computation intensive since they have a complexity of O(N), where N is the length of the overall impulse response.
An object of the present invention is, therefore, to propose a method for detecting multipath signals and a corresponding RAKE receiver whose data processing is less complex than the known detection methods, thus allowing fundamentally shorter adjustment times and a reduced processor capacity requirement.